The human complement factor I gene (IF) was cloned from a flow-sorted cosmid library. The gene spans 63 kb and comprises 13 exons. The first exon, which encodes the leader sequence and 5' untranslated region, is separated from the body of the gene by a large intron of 36 kb. Factor I is a mosaic protein, and there is a correlation between the genomic organization and the modular structure of the protein. The second exon encodes a module found only in complement C6 and C7 (FI/C6/C7); the third and fourth exons encode a single CD5 domain; and the fifth and sixth exons each encode a low-density lipoprotein receptor module. Two very small exons, 21 and 36 bp, then separate the first six exons from the last five that encode the serine protease domain of factor I. Within the serine protease gene family factor I has a unique genomic structure, but it bears a much closer resemblance to trypsin than it does to the other complement system serine proteases, factor B, C2, and C1r/C1s.